The present invention relates to fume hoods and, in particular, to an apparatus and method for monitoring air entry velocity into an air chamber of the fume hood.
Fume hoods are well known in the art. Generally, each fume hood includes walls defining an air containment chamber and one or more sash doors through which materials within the air containment chamber are handled. A negative pressure source is connected with the air chamber for withdrawing air from the chamber that is contaminated with noxious or hazardous fumes given off by the materials kept within the chamber. To preclude the fumes from escaping through an open sash door, a sufficient source of negative pressure is maintained to create a sufficient airflow through the open sash door to sweep all of the fumes away toward the negative pressure source. Additionally, vents and the like are also provided for admitting air from outside of the chamber into the chamber for sweeping the fumes away toward the negative pressure source when the sash doors each are closed.
It is conventional practice to monitor the airflow through open sash doors, vents and the like (also referred to as xe2x80x9cair entry velocityxe2x80x9d or xe2x80x9cface velocityxe2x80x9d) by measuring the velocity of airflow through a small air passageway provided in a wall of the fume hood defining the air chamber. Such monitoring safeguards against otherwise unnoticed ruptures or leaks in ductwork or a fume hood wall, and against exhaust fan failure. If undetected, any of these eventualities could reduce air entry velocity into the air chamber and permit contaminated air to escape from the fume hood.
The conventional airflow sensor is thermal in nature and typically is disposed outside of the fume hood air chamber and directly admits air into the air chamber through the small air passageway disposed in the fume hood wall. In this regard, the air passageway consists of a single entry orifice disposed in the air chamber wall and an air tube connects the sensor with the orifice in airtight communication.
Velocity through an airflow sensor is either equal to or proportional to air entry velocity into the air chamber. In normal operating conditions, acceptable air entry velocity is between 80-200 FMP. An alarm also is provided in conjunction with the sensor and is preset to go off in the event that the airflow velocity drops to 60-90 FMP, depending on the alarm setpoint, which is taken to indicate a rupture, leak or other problem with the fume hood.
The conventional location of the entry point of the airflow into the air chamber through the sensor is believed to be unsuitable to yield a representative monitoring point reflective of overall average air entry velocity into the air chamber. In particular, since the entry point of the air passageway is necessarily located on one side of the air chamber, an open sash door proximate the entry point will more greatly influence airflow through the sensor than would an open sash door of the same size on the opposite side of the air chamber. Consequently, the current monitoring technique is not as accurate and consistent as is otherwise desired; the sensor readings are primarily only representative of the local vicinity of the air passageway entry point within the air chamber, and not of the overall air entry velocity of the fume hood extending from side-to-side of the air chamber.
Accordingly, the present invention provides a fume hood in which a sensor more accurately monitors overall air entry velocity into the air chamber.
Briefly described, the fume hood of the present invention includes a housing having walls defining a chamber for confinement of contaminated air therein. One of the walls defines an air passageway for airflow from outside of the chamber into the chamber, and a sensor is located outside of the chamber and is disposed in airtight communication with the air passageway for measuring airflow through the air passageway. In a feature of the present invention, a conduit extends between and connects the sensor in airtight communication with the air passageway.
In accordance with the present invention, an elongate, perforated tubular member is supported within the chamber and includes a first end disposed in surrounding relation to the air passageway. A second, distal end of the tubular member is substantially closed off from the air within the chamber. Accordingly, airflow through the air passageway generally must pass through one of many perforations in the tubular member to gain access into the air chamber.
In features of the present invention: the tubular member comprises a generally linear, horizontally extending rigid cylindrical tube; the tubular member extends across an entire width of the chamber and is mounted in orthogonal relation to opposed walls of the housing defining the chamber; and the perforations are generally aligned in a single row along the tubular member with each perforation generally facing in the same radial direction.
The present invention also includes the method of monitoring air entry velocity into an air chamber of a fume hood, including the steps of: covering an air passageway in a wall defining the air chamber of the fume hood with a perforated tubular member such that airflow from outside of the fume hood through an air passageway generally must pass through a perforation in the tubular member to gain access into the air chamber; and thereafter measuring airflow velocity through the air passageway.